Sol-gel process using porous mold

ABSTRACT

A sol-gel process for producing dry porous gel monoliths, e.g., silica glass monoliths, in which the successive process steps of gelling, aging and drying all occur within a mold formed of a porous material, e.g., graphite. The mold is inert to the gel solution and it has sufficient strength to withstand the temperatures and pressures encountered during the process, yet it has sufficient porosity to facilitate the escape of liquid from the gel pores directly through the mold, itself. The mold and gel thereby can remain within a sealed autoclave during these process steps, and mechanical handling of the mold and the gel are minimized. This substantially enhances the process&#39; efficiency. Alternatively, the mold can have a non-porous inner skin.

This is a continuation-in-part of provisional application Ser. No.60/103,346, filed Oct. 7, 1998.

BACKGROUND OF THE INVENTION

This invention relates generally to sol-gel processes for producing drygel monoliths that subsequently can be sintered into glass articles and,more particularly, relates to sol-gel processes of this kind using moldsspecially configured to enhance the process' effectiveness.

Substantial efforts have recently been expended in developing improvedsol-gel processes for fabricating high-purity monolithic articles ofglass. In such processes, a desired solution, i.e., a sol, containingglass-forming compounds, solvents, and catalysts, is poured into a moldand allowed to react. The solution typically includes tetraethylorthosilicate, water, an alcohol, and an acid and/or base catalyst.Following hydrolysis and condensation reactions, the sol forms a porousmatrix of solids, i.e., a gel. With aging, the gel shrinks in size byexpelling fluids from the pores of the gel. The wet gel is then dried ina controlled environment, typically by removing the gel from the moldand placing it into an autoclave for subcritical or supercriticalheating. The dried gel then is sintered into a solid monolith.

Advantages of the sol-gel process include chemical purity andhomogeneity, flexibility in the selection of compositions, the abilityto process at relatively low temperatures, and the producing ofmonolithic articles close to their final desired shapes, therebyminimizing finishing costs.

The efficiency of the process can be enhanced if the steps of gelling,aging and drying all are carried out within a single chamber and withoutthe need to remove the gel from the mold. The need to remove the gelfrom the mold at an intermediate step of the process not only requiresmechanical handling of the fragile gel and mold, but also lengthens theprocessing time. This is because removing the gel from the moldfollowing the step of aging can be performed only after the gel hascooled to room temperature from its aging temperature, e.g., 60° C.

An important factor bearing on the ability to perform the entire sol-gelprocess without removing the gel from the mold is the nature of thematerial from which the mold is formed. The ideal mold material shouldhave good release characteristics, such that the fragile monolithic gelcan be removed from the mold without damage.

The mold material also should be inert to attack from chemicals used inthe sol-gel process, e.g., acid catalysts such as hydrochloric acid(HCl) and base catalysts such as ammonium hydroxide (NH₄ OH). Thisrequirement effectively precludes the use of molds formed of metal,because metal impurities could be leached from the mold and trapped inthe gel, thus being retained in the glass monolith. Metal impuritiesretained within a glass monolith are particularly undesirable, becausethey can reduce the transmission of ultraviolet light. Such leachingalso can reduce the mold's life span.

If the gel is to be dried while still located within the mold, the moldmaterial must be able to withstand typical drying temperatures, e.g.,200° C. and above. This means that the mold must not decompose at suchtemperatures and it should not deform when repeatedly cycled betweenroom temperature and the maximum drying temperature. This requirementeffectively excludes the use of molds formed of common polymericmaterials such as polymethyl pentane and Teflon, which have softeningtemperatures substantially lower than 200° C.

Some refractory ceramics, e.g., silicon carbide, boron nitride andcarborundum, can survive the required high temperatures and arereasonably inert, making them suitable for use as mold materials.However, such materials are difficult and expensive to machine into therequired mold shapes. In addition, these materials do not generally havegood release characteristics, and gels can sometimes adhere to moldsmade of these materials.

It should therefore be appreciated that there is a need for a sol-gelprocess in which the steps of gelling, aging and drying all are carriedout without removing the material from the mold. The present inventionfulfills this need and provides further related advantages.

SUMMARY OF THE INVENTION

The present invention resides in an improved sol-gel process forproducing a dry porous gel monolith, in which the process steps ofgelling, aging and drying all are carried out while the gel remainswithin a mold, thus substantially reducing mechanical handling of thegel and mold and substantially enhancing the process' efficiency. Moreparticularly, the process incorporates steps of 1) placing a solutioninto a mold formed of a porous material such as graphite, siliconcarbide, titanium carbide, or tungsten carbide, 2) allowing the solutionto gel within the mold, 3) drying the gel within the mold, and 4)removing the dried gel from the mold to obtain the gel monolith.

The process has particular advantages when used to produce gel monolithsin the form of high-purity silica. In such applications, the solutionconsists essentially of tetraethyl orthosilicate, an alcohol, deionizedwater, and an acid catalyst and/or a base catalyst, in prescribedrelative proportions. In addition, the process can further include astep of aging the gel within the mold, before the step of drying, and afurther step of sintering the dried gel after the step of removing.

In one configuration, the mold is configured to be substantiallyhomogeneous, with sufficient porosity to allow liquid present in thepores of the gel to escape therethrough during the step of drying. In analternative configuration, the mold is configured to have a porous bodywith a substantially non-porous inner skin. In that alternativeconfiguration, the pore liquid escapes from the narrow annular spacebetween the graphite mold and the gel. In both configurations, the moldpreferably has a substantially uniform thickness in the range of 3 to 5mm. In the case of molds formed of graphite, the graphite preferably hasa bulk density of about 1.75 gm/cm³ and a porosity in the range of about10 to 15%.

In other more detailed features of the invention, the steps of allowingthe solution to gel, aging the gel, and drying the gel all occur whilethe solution and gel remain located within the mold. In addition, thesesteps all occur while the mold is located within an autoclave. The stepof drying the gel in the autoclave can occur either under subcritical orsupercritical conditions.

Other features and advantages of the present invention should becomeapparent from the following description of the preferred process, takenin conjunction with the accompanying drawing, which illustrates, by wayof example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE is a sectional elevational view of a porous graphite moldlocated within an autoclave and specially configured for use in asol-gel process for producing silica monoliths.

DESCRIPTION OF THE PREFERRED PROCESS

With reference now to the illustrative FIGURE, there is shown a mold 11located within an autoclave 13, for use in a sol-gel process forproducing crack-free silica monoliths. The mold has a size and shapesubstantially the same as that desired for the monolith to be produced,and it is formed of a porous graphite material, which enables thesuccessive sol-gel process steps of gelling, aging and drying all to becarried out without the need to remove the gel from the mold.

In an initial step of the process, a suitably hydrolyzed siliconalkoxide sol is poured into the mold 11 and allowed to gel at roomtemperature for about 16 hours. One suitable sol can comprise tetraethylorthosilicate (TEOS), ethanol, deionized water, hydrochloric acid, andammonia, in relative molar proportions of about 1:1.5:4:0.0001:0.0003,respectively. Alternatively, tetramethyl orthosilicate (TMOS) can besubstituted for the TEOS.

After the sol has gelled to form a gel 15, a suitable amount of freshliquid 17 is added to the mold 11, to top off and fully immerse the gel.This helps to prevent the gel from cracking during the subsequent stepof aging. It also eliminates the need to use a lid on the mold, thussimplifying the process. The composition of this added liquid preferablyis the same as the pore liquid contained within the gel.

The graphite mold 11 with the immersed gel 15 is then introduced intothe autoclave 13, where it is elevated above the floor of the autoclaveon a support 19, and the temperature within the autoclave is raised toabout 6° C. over a span of about six hours, and maintained at thattemperature for about 42 hours. During this aging step, a saturatedambient is maintained within the autoclave by providing an excess ofpore liquid on the floor of the autoclave, as indicated by the referencenumeral 21. The aging step effectively increases the gel's average poresize and strengthens the gel, so as to reduce the gel's susceptibilityto cracking during the subsequent step of drying.

After the aging step has been completed, a drying solvent, e.g.,isopropanol, is introduced into the autoclave 13, and the temperatureand pressure within the autoclave are raised according to prescribedprofiles. Drying can be achieved using both subcritical andsupercritical procedures. One suitable subcritical drying procedure isdisclosed in U.S. Pat. No. 5,473,826, which is incorporated byreference.

Significantly, the step of drying is performed without first removingthe gel 15 from the mold 11. The mold's porosity facilitates this dryingby allowing the liquid contained within the gel's pores to escapedirectly through the mold, itself. In the preferred process, the mold ishomogeneous and formed of an isomolded, fine-grained graphite materialhaving high thermal conductivity and high strength. One suitablegraphite material is available from Le Carbone-Lorraine, under the nameGraphite Grade 2191. It has a bulk density of about 1.74 gm/cm³, and ithas a porosity of about 13%.

Alternatively, the graphite mold 11 can incorporate a non-porous,mirror-like portion defining its inner skin or wall. In that alternativecase, the liquid evaporates from the narrow annular space between themold's inner wall and the wet gel's outer surface. In addition, the moldcan be formed of porous carbide materials such as silicon carbide,titanium carbide, tungsten carbide, and mixtures thereof All of thesematerials are inert to the alkoxide solution and are able to withstanddrying temperatures of up to 300° C.

The mold 11 has a size and shape substantially the same as that desiredfor the monolith to be produced, and it preferably has a uniformthickness in the range of about 3 to 5 mm. A minimum thickness of 3 mmwill ensure that the mold has adequate structural integrity, and amaximum thickness of 5 mm will ensure that the mold will not undulyinhibit the escape of pore liquid during the step of drying. When a moldhaving a non-porous inner skin is used, the inner skin preferably has auniform thickness less than about 1 mm.

Presented below is a more detailed description of the preferred processfor producing dry gel monoliths using a mold 11 formed of a porousgraphite material. Although the process is described with particularreference to silica gel monoliths formed of silica, it will beappreciated that the use of a mold formed of a graphite material canenhance the efficient production of other aerogel or xerogel monolithsas well.

Mold Cleaning--For the graphite mold 11 to have the desired releaseproperties, it is important that it be thoroughly cleaned of particlesremaining inside the pores of the mold from a previous casting. This canbe achieved by first immersing the mold in a dilute 7% hydrofluoric acid(HF) solution for 30 minutes, followed by an ultrasonic HF bath for 20minutes. The mold then is immersed in deionized water for 30 minutes,followed by two successive ultrasonic baths in deionized water, for 20minutes each. Finally, the mold is placed in a clean drying oven to dry.

Sol Preparation and Casting--The prescribed sol is mixed in a reactorvessel that has been appropriately cleaned with a dilute 7% HF solution.In the preferred process, for producing a silica gel monolith, this solincorporates TEOS, ethanol, deionized water, HCl, and NH₄ OH, inrelative molar proportions of about 1:1.5:4:0.0001:0.0003, respectively.The sol is then transferred to the previously cleaned graphite mold 11,while located in a Class 100 laminar flow hood.

Gelation--The graphite mold 11, with the cast sol, is then introducedinto the autoclave 13, for gelation. After the autoclave has beensealed, the sol is allowed to gel at room temperature for 16 hours. Atthat time, an aging solution having a composition of about 88% ethanoland 12% deionized water is pumped into the autoclave, to substantiallyfill the autoclave, The autoclave then is drained, leaving the moldtopped off with the liquid and further leaving sufficient liquid 21remaining on the floor of the autoclave to maintain a saturationpressure of 9 psi at 60° C. This topping liquid is added to inhibitcracking of the gel during the subsequent aging step and further toeliminate the need for a lid on the mold, thus simplifying the process.The FIGURE depicts the autoclave and mold at this stage of the process.

Aging--After the graphite mold 11 has been topped off with theprescribed aging solution, the temperature of the autoclave is linearlyramped up to 60° C. over a time span of 6 hours, and the temperature isthen maintained at that temperature for a further 42 hours. Thiscompletes an in-situ aging step, in which the average pore size in thegel 15 is increased to a point where the gel can properly avoid crackingduring the subsequent drying step.

Drying--After the step of aging has been completed, the aging solution21 that remains on the floor of the autoclave 13 is drained away andpure isopropanol is pumped into the autoclave at a pressure of 9 psi,while the temperature is maintained at 60° C. About 1500 milliliters ofisopropanol are added for an autoclave having a volume of 20 liters. Thetemperature of the autoclave is then linearly ramped up to 240° C. andallowed to equilibrate at that temperature for one hour. This typicallyincreases the pressure to about 620 psi. The pressure within theautoclave is released over a period of about five hours, while the 240°C. temperature is maintained. Finally, the autoclave is cooled to roomtemperature and the graphite mold 11 and gel 15 are removed.

The dried, crack-free monolithic aero gel 15 then can be readily removedfrom the graphite mold 11. Following sintering, a pure silica monolithof optical quality is obtained. The mold then can be used again toproduce further dry porous gel monoliths, if it is appropriately cleanedin the manner described above.

The special use of a mold 11 formed of graphite substantially enhancesthe efficiency of the sol-gel process. The use of this material isparticularly effective, because it allows the successive sol-gel processsteps of gelling, aging and drying all to be carried out without theneed to remove the gel from the mold. The porosity of the moldfacilitates the desired solvent exchange and also the escape of the poreliquid directly through the mold, itself, during the drying step.

Graphite also is a particularly advantageous material for the mold 11,because it can withstand temperatures greater than 300° C., under thespecified drying conditions, without deforming or decomposing. Inaddition, graphite does not chemically interact with the specified sol,and it exhibits good mold release properties under controlledconditions. If any graphitic carbon is incidentally introduced as animpurity into the gel monolith, it can be readily removed during thesintering operation. Finally, molds formed of graphite are substantiallyless expensive than are molds formed of other conventional materials.

Although the invention has been described in detail with reference tothe presently preferred process, those of ordinary skill in the art willappreciate that various modifications can be made without departing fromthe invention. Accordingly, the invention is defined only by thefollowing claims.

We claim:
 1. A process for producing a dry, porous, silica gel monolithcomprising the steps of:placing a solution into a mold formed of aporous material that is inert to the solution and that is formedprincipally of a material selected from the group consisting ofgraphite, carbides and mixtures thereof, wherein the solution consistsessentially of the following components, in prescribed relativeproportionstetraethyl orthosilicate or tetramethyl orthosilicate, analcohol, deionized water, and an acid catalyst and/or a base catalyst;allowing the solution to gel within the mold; drying the gel within themold; and removing the dried gel from the mold to produce a dry, porous,silica gel monolith.
 2. A process as defined in claim 1, wherein themold used in the steps of placing, allowing, drying and removing isformed principally of a material selected from the group consisting ofgraphite, silicon carbide, titanium carbide, tungsten carbide, andmixtures thereof.
 3. A process as defined in claim 2, wherein the moldused in the steps of placing, allowing, drying and removing issubstantially homogeneous and formed principally of graphite.
 4. Aprocess as defined in claim 1, wherein the step of drying comprises astep of elevating the temperature of the gel sufficiently to dry the gelunder supercritical conditions.
 5. A process as defined in claim 1,wherein the step of drying comprises a step of elevating the temperatureof the gel sufficiently to dry the gel under subcritical conditions. 6.A process as defined in claim 1, wherein:the process further comprises astep of sintering the dried gel monolith after the step of removing, toproduce the silica monolith.
 7. A process as defined in claim 6, andfurther comprising a step of aging the gel within the mold, before thestep of drying.
 8. A process as defined in claim 1, wherein the solutionused in the step of placing further comprises the following components,in prescribed relative proportions:colloidal silica; and a dispersant.9. A process as defined in claim 1, wherein the mold used in the stepsof placing, allowing, drying and removing has sufficient porosity toallow liquid present in pores of the gel to escape therethrough.
 10. Aprocess as defined in claim 1, wherein the mold used in the steps ofplacing, allowing, drying and removing has a bulk density of about 1.75gm/cm³ and a porosity in the range of about 10 to 15%.
 11. A process asdefined in claim 1, wherein the mold used in the steps of placing,allowing, drying and removing has a substantially uniform thickness inthe range of 3 to 5 mm.
 12. A process as defined in claim 1, wherein:theprocess further comprises a step of aging the gel within the mold,before the step of drying; and the steps of allowing the solution togel, aging the gel, and drying the gel all occur while the solution andgel remain located within the mold.
 13. A process as defined in claim12, wherein the steps of allowing the solution to gel, aging the gel,and drying the gel all occur while the mold is located within anautoclave.
 14. A process as defined in claim 13, wherein the step ofaging includes a step of adding a topping liquid to the mold, whereinthe topping liquid has substantially the same composition as the poreliquid.
 15. A process for producing a monolithic article of high-puritysilica, comprising the steps of:mixing an alkoxide solution thatconsists essentially of the following components, in prescribed relativeproportionstetraethyl orthosilicate or tetramethyl orthosilicate, analcohol, deionized water, and an acid catalyst and/or a base catalyst;placing the alkoxide solution into a mold formed of a porous graphitematerial having a porosity in the range of about 10 to 15% and having asubstantially uniform thickness in the range of 3 to 5 mm; allowing thealkoxide solution to gel within the porous graphite mold; aging the gelwithin the porous graphite mold; drying the gel within the porousgraphite mold; removing the dried gel from the porous graphite mold; andsintering the dried gel to produce a monolithic article of high-puritysilica glass.
 16. A process as defined in claim 15, wherein the steps ofallowing the alkoxide solution to gel, aging the gel, and drying the gelall occur while the solution and gel remain located within the porousgraphite mold.
 17. A process as defined in claim 16, wherein the stepsof allowing the alkoxide solution to gel, aging the gel, and drying thegel all occur while the porous graphite mold is located within anautoclave.
 18. A process for producing a dry, porous, silica gelmonolith comprising the steps of:placing a solution into a mold formedof a material that is inert to the solution and that is formedprincipally of a material selected from the group consisting ofgraphite, carbides, and mixtures thereof, wherein the mold has a porousbody and a non-porous inner skin, and wherein the solution used in thestep of placing consists essentially of the following components, inprescribed relative proportionstetraethyl orthosilicate or tetramethylorthosilicate, an alcohol, deionized water, and an acid catalyst and/ora base catalyst; allowing the solution to gel within the mold; dryingthe gel within the mold; and removing the dried gel from the mold toproduce a dry, porous, silica gel monolith.
 19. A process as defined inclaim 18, wherein the porous body of the mold used in the steps ofplacing, allowing, drying and removing is formed principally of amaterial selected from the group consisting of graphite, siliconcarbide, titanium carbide, tungsten carbide, and mixtures thereof.
 20. Aprocess as defined in claim 18, wherein the porous body of the mold usedin the steps of placing, allowing, drying and removing is formedprincipally of graphite.
 21. A process as defined in claim 18, whereinthe step of drying comprises a step of elevating the temperature of thegel sufficiently to dry the gel under supercritical conditions.
 22. Aprocess as defined in claim 18, wherein the step of drying comprises astep of elevating the temperature of the gel sufficiently to dry the gelunder subcritical conditions.
 23. A process as defined in claim 18,wherein:the process further comprises a step of sintering the dried gelmonolith after the step of removing, to produce the silica monolith. 24.A process as defined in claim 23, and further comprising steps of agingthe gel within the mold, before the step of drying.
 25. A process asdefined in claim 18, wherein the solution used in the step of placingfurther comprises the following components, in prescribed relativeproportions:colloidal silica; and a dispersant.
 26. A process as definedin claim 18, wherein the mold used in the steps of placing, allowing,drying and removing has a substantially uniform thickness in the rangeof 3 to 5 mm.
 27. A process as defined in claim 26, wherein thenon-porous inner skin of the mold has a thickness of less than about 1mm.
 28. A process as defined in claim 18, wherein:the process furthercomprises a step of aging the gel within the mold, before the step ofdrying; and the steps of allowing the solution to gel, aging the gel,and drying the gel all occur while the solution and gel remain locatedwithin the mold.
 29. A process as defined in claim 28, wherein the stepsof allowing the solution to gel, aging the gel, and drying the gel alloccur while the mold is located within an autoclave.
 30. A process asdefined in claim 29, wherein the step of aging includes a step of addinga topping liquid to the mold, wherein the topping liquid hassubstantially the same composition as the pore liquid.
 31. A process asdefined in claim 18, wherein the non-porous inner skin has a mirror-likesurface.